Continuous Flow Synthesis and Kinetic Study of Diphenyl Sulfoxide in a Microreactor

The oxidation of diphenyl sulfide (DPS) by hydrogen peroxide (H₂O₂) to synthesize diphenyl sulfoxide (DPSO) is extremely exothermic and has a high thermal risk. When thermal runaway happens, it may lead to equipment damage or even explosions. Therefore, in this work, a microreactor was adopted to reduce reaction thermal risk and process conditions were optimized. Phosphotungstic acid (PTA) was used as the catalyst, and the effects of process conditions, including reaction temperature, residence time, catalyst concentration, and molar ratio on the conversion and yield were systematically investigated. The results showed that the DPSO yield could reach up to 84.3% under the condition of 0.75% catalyst loading, 25 min residence time, 70 °C reaction temperature, and H₂O₂-DPS molar ratio of 2. Then, apparent reaction kinetics were studied, and a kinetic model was established and validated. By varying the initial concentrations of H₂O₂ and DPS, the reaction was determined to be of second-order, with an activation energy of 57.5 kJ·mol^-1 and a pre-exponential factor of 2.96 × 10⁷ mol^-1·L·min^-1. Furthermore, the temperature distribution along the microreactor was estimated by combining the thermal equilibrium with the reaction kinetics. The results indicated that in a 1/16 in. microreactor, the reaction was nearly isothermal. Temperature distributions were also predicted for microreactors with different diameters and materials. It was demonstrated that the reaction could be safely scaled up to a 3/8 in. microreactor at a reaction temperature of 55 °C, with the maximum temperature rise remaining below 5 °C and no decline in DPSO yield. This study provided a convenient method to guide the safe sizing-up of the reaction in flow reactors.